MODULATION OF RIGOR AND MYOSIN ATPASE ACTIVITY IN RAT CARDIOMYOCYTES

Ischaemic myocardium undergoes calcium-independent contracture at mill imolar tissue ATP, though in actomyosin solutions ATP must be reduced to micromolar before rigor complexes form. This contracture is associa ted with myosin ATPase activity that may contribute to tissue de-energ ization. Here we used isolated rat cardiomyocytes permeabilized with d igitonin to analyse in parallel how rigor and myosin ATPase activity a re modulated by metabolic conditions that develop during ischaemia. At pH 7.1 and 37 degrees C rigor and myosin ATPase showed co-ordinated b ell-shaped dependence on ATP concentration over 3-1000 mu M. Rigor, bu t not myosin ATPase, was inhibited by acidosis (pH 6.2), indicating re duced efficiency of cross-bridge cycling, while both parameters were s timulated by ADP (less than or equal to 1 mM) and unaffected by inorga nic phosphate (P-i, 30 mM), AMP, Mg2+, lactate or inhibition of adenyl ate kinase with diadenosine pentaphosphate. Combined acidosis and high ADP inhibited rigor, while P-i attenuated the enhancement of rigor by ADP. Thus, rigor complex formation activates myosin ATPase in the int act myofilament array, modulated by ADP, P-i and acidosis in the range s that occur in ischaemia. There was no evidence that adenylate kinase might attenuate falling ATP/ADP ratio at the myofilaments. In combina tion these effects are sufficient to resolve the apparent discrepancy between ATP concentrations triggering rigor in actomyosin and onset of contracture in ischaemic myocardium. Since rigor contracture activate s myosin ATPase it is likely to exacerbate ATP depletion and thereby l imit vital cell functions. This positive feedback is consistent with t he abrupt depletion of ATP observed in individual cardiomyocytes under going deenergization contracture. (C) 1998 Academic Press.